Guinea pigs with various conditioning treatments are being exposed to the inhalation of BeSO4 aerosol. The conditioning treatments include induction of hypersensitivity with intradermal BeSO4; passive transfer of hypersensitivity with lymphocytes; and desensitization with antilymphocyte serum. Appropriate controls are included. The pulmonary response after inhalation of BeSO4 in the variously conditioned groups is being compared and correlated with the prevailing degree of hypersensitivity as assayed in the macrophage migration inhibition test. This research is expected to clarify the nature of the immunologic component in the etiology of pulmonary berylliosis, and to suggest means by which the development of the disease could be inhibited.